Printing complex electronic circuits

ABSTRACT

A programmable circuit includes an array of printed groups of microscopic transistors or diodes. The devices are pre-formed and printed as an ink and cured. The devices in each group are connected in parallel so that each group acts as a single device. In one embodiment, about 10 devices are contained in each group so the redundancy makes each group very reliable. Each group has at least one electrical lead that terminates in a patch area on the substrate. An interconnection conductor pattern interconnects at least some of the leads of the groups in the patch area to create logic circuits for a customized application of the generic circuit. The groups may also be interconnected to be logic gates, and the gate leads terminate in the patch area. The interconnection conductor pattern then interconnects the gates for form complex logic circuits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/204,800, filed Mar. 11, 2014, which is based on U.S. provisionalapplication Ser. No. 61/785,292, filed Mar. 14, 2013, by WilliamJohnstone Ray et al., assigned to the present assignee and incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to printing pre-formed, microscopic semiconductordevices, such as transistors and diodes, in separate groups on asubstrate, where the randomly distributed devices in each group areconnected in parallel, and interconnecting the groups to create morecomplex circuits, such as logic circuits.

BACKGROUND

It is known, by the present assignee's own work, how to form and printmicroscopic 2-terminal vertical light emitting diodes (LEDs), with theproper orientation, on a conductive substrate and connect the LEDs inparallel to form a light sheet. Details of such printing of LEDs can befound in US application publication US 2012/0164796, entitled, Method ofManufacturing a Printable Composition of Liquid or Gel Suspension ofDiodes, assigned to the present assignee and incorporated herein byreference.

FIG. 1 is a cross-sectional view of a layer of LEDs 16 that may beprinted using the following process. Each LED 16 includes standardsemiconductor GaN layers, including an n-layer, and active layer, and ap-layer.

An LED wafer, containing many thousands of vertical LEDs, is fabricatedso that the bottom metal cathode electrode 18 for each LED 16 includes areflective layer. The top metal anode electrode 20 for each LED 16 issmall to allow almost all the LED light to escape the anode side. Acarrier wafer, bonded to the “top” surface of the LED wafer by anadhesive layer, may be used to gain access to both sides of the LED formetallization. The LEDs 16 are then singulated, such as by etchingtrenches around each LED down to the adhesive layer and dissolving theexposed adhesive layer or by thinning the carrier wafer.

The microscopic LEDs are then uniformly infused in a solvent, includinga viscosity-modifying polymer resin, to form an LED ink for printing,such as screen printing or flexographic printing.

If it is desired for the anode electrodes 20 to be oriented in adirection opposite to the substrate 22 after printing, the electrodes 20are made tall so that the LEDs 16 are rotated in the solvent, by fluidpressure, as they settle on the substrate surface. The LEDs 16 rotate toan orientation of least resistance. Over 90% like orientation has beenachieved.

In FIG. 1, a starting substrate 22 is provided. If the substrate 22itself is not conductive, a reflective conductor layer 24 (e.g.,aluminum) is deposited on the substrate 22 such as by printing. Thesubstrate 22 may be thin and flexible.

The LEDs 16 are then printed on the conductor layer 24 such as byflexography, where a pattern on a rolling plate determines thedeposition for a roll-to-roll process, or by screen printing with asuitable mesh to allow the LEDs to pass through and control thethickness of the layer. Because of the comparatively low concentration,the LEDs 16 will be printed as a monolayer and be fairly uniformlydistributed over the conductor layer 24.

The solvent is then evaporated by heat using, for example, an infraredoven. After curing, the LEDs 16 remain attached to the underlyingconductor layer 24 with a small amount of residual resin that wasdissolved in the LED ink as a viscosity modifier. The adhesiveproperties of the resin and the decrease in volume of resin underneaththe LEDs 16 during curing press the bottom LED electrode 18 against theunderlying conductor 24, making ohmic contact with it.

A dielectric layer 26 is then printed over the surface to encapsulatethe LEDs 16 and further secure them in position.

A top transparent conductor layer 28 is then printed over the dielectriclayer 26 to electrically contact the electrodes 20 and is cured in anoven appropriate for the type of transparent conductor being used.

If needed to spread current, metal bus bars 30-33 are then printed alongopposite edges of the conductor layers 24 and 28 and electricallyterminate at anode and cathode leads (not shown), respectively, forenergizing the LEDs 16. The bus bars 30-33 will ultimately be connectedto a positive or negative driving voltage.

FIG. 2 is a top down view of FIG. 1. The cross-section of FIG. 2 is ahorizontal bisection of FIG. 3. The locations of the LEDs 16 in theprinted layer are random.

If a suitable voltage differential is applied to the anode and cathodeleads, all the LEDs 16 with the proper orientation will be illuminated.FIG. 1 shows a light ray 38.

The above process is strictly for use with 2-terminal devices having atop electrode and a bottom electrode, since the locations of the LEDs onthe substrate are random, and the LEDs can only be interconnected bysandwiching the LEDs between two conductive layers of any thickness.Further, the above process is strictly for forming an array of LEDs forgenerating light. The LEDs are not intended to perform any type of logicfunction since the array of parallel-connected LEDs simply forms asingle diode.

It would be desirable to adapt the above-described printing/curingprocess to create complex printed electrical circuits involving3-terminal transistors, diodes, and possibly additional types ofcomponents, to perform logic functions.

SUMMARY

The present invention is broadly related to printing pre-formed,microscopic (e.g., dimensions between 10-200 microns) electronicdevices, including transistors and diodes, in small separate groups on asubstrate, such as a flex-circuit. Each group may contain, for example,about 10 of the devices. The devices in each group are connected inparallel using printed conductor layers.

Each group acts as a single device (e.g., a single transistor or asingle diode), since the same devices are connected in parallel in eachgroup. At any time after the groups are formed, the groups are theninterconnected (programmed) to form a customized circuit, such as alogic circuit for performing a specified function.

In one embodiment, the printed devices are transistors or diodes, andthe programming step forms a plurality of logic gates. In anotherembodiment, the substrate is initially processed to create an array oflogic gates from the groups, and a subsequent “programming” stepcustomizes the substrate by interconnecting the gates to form a complexlogic circuit. Therefore, the printed substrate may form a programmablegate array.

In one embodiment, the “programming” to create the circuit is performedby forming a hydrophobic mask on the substrate, defining theinterconnection pattern, and then depositing a conductive material toform interconnecting metal traces on the substrate. In anotherembodiment, the interconnecting traces are directly printed on thesubstrate by flexography or screen printing.

The groups of devices may all be the same devices (e.g., transistors) ora variety of devices (e.g., transistors and diodes). The circuits may beother than logic circuits, such as control circuits, switching circuits,analog circuits, etc.

Many types of electrical components use three terminals, such asMOSFETs, bipolar transistors, JFETs, thyristors, silicon controlledrectifiers, etc. Conventional ones of such components typically havethree terminals on the top, for lateral devices, or two terminals on topand one on the bottom, for vertical devices. It is known to form thinfilm transistors by printing the various transistor layers over asubstrate, but the performance of such printed transistors is poor dueto the difficulty of printing a single crystal. If transistors (or other3-terminal devices) could be more conventionally formed in asemiconductor wafer and then singulated to create microscopic devicesfor printing as an ink, the quality of the devices may be state of theart. However, heretofore it is not known how to design such devices orto interconnect such 3-terminal microscopic devices after printing toperform complex functions.

In one embodiment, a semiconductor (e.g., silicon) wafer of 3-terminaldevices, such as transistors, is formed. The transistors are formed inthe wafer to have a bottom electrode, a top electrode, and anintermediate electrode that is located on a shelf somewhere between thetop and bottom of the device. The starting wafer is ultimately affixedto a carrier wafer, by an adhesive, to gain access to both surfaces ofthe transistors when fabricating them.

The transistors are singulated into individual transistors by formingtrenches around each transistor, such as to form hexagonal devices. Thetrenches extend down to the adhesive layer, and the adhesive layer isdissolved in a solution, releasing all the transistors from the carrierwafer.

The transistors are then uniformly mixed into a solution to form an ink.The shapes of the transistors cause a vast majority them to be printedin the desired orientation on a substrate.

The transistors are then printed to form an array of groups oftransistors on associated first conductor layer portions over asubstrate, and the ink is cured (heated and evaporated), so that thebottom electrode of each transistor makes ohmic contact to these firstconductor layer portions. The transistors will be printed as a loosemonolayer due to the relatively low density of the transistors in thesolution. The printing of any layers in the product may be byflexography (particularly suited for a roll-to-roll process), screenprinting (particularly suited when forming flat sheets), or other typesof printing.

A first dielectric layer is then printed over the first conductor layerportions. The first dielectric layer does not cover the intermediateelectrode. Second conductor layer portions, aligned with the firstconductor layer portions, are then printed, which contact theintermediate electrode but do not cover the top electrode. The variousthin printed layers self-planarize by a strong surface tension so thatthe layer does not cover any features “above” the thickness of thelayer. Alternatively, the layers may be blanket etched after curing toexpose any electrodes.

A second dielectric layer is then printed over the second conductorlayer portions but not over the top electrode. Top (third) conductorlayer portions are then printed to contact the top electrode of thetransistors in each group.

Therefore, the top electrodes of the transistors are connected inparallel, the bottom electrodes are connected in parallel, and theintermediate electrodes (or a subset of them) are connected in parallelfor conducting a wide range of currents.

As previously mentioned, the groups may then be interconnected in aprogramming step to form logic gates or more complex circuits.

Instead of transistors, microscopic vertical diodes may be printed, andonly two conductor layers are needed to connect the diodes in each groupin parallel.

For simple passive devices such as resistors, the resistive materialitself (rather than printing an ink containing individual resistors) maybe printed in a small area, and the resistance is determined by where aconductor contacts the resistor along its length.

Different areas of the substrate may be printed with different devicesor the same devices, and the devices in each area are connected inparallel. Therefore, each area is essentially a single device. Theconductor layers terminate in connector areas on the substrate next toeach area.

In one embodiment, the substrate may have a designated “patch” areawhere the interconnections of the groups or the gates are made. Thissimplifies the design of the programmed interconnection, since the patcharea may be optimized for the programming step.

The devices are formed such that, if some of the devices in a group areprinted upside down or make a poor connection, there is no adverseeffect on the functions of the properly orientated devices in the group.

The printing process may use a roll-to-roll process at atmosphericpressures. The cost of the printed programmable substrates is much lowerthan the cost of comparable programmable substrates formed usingconventional techniques.

Other embodiments are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a loose monolayer of printed, microscopicvertical LEDs that may be formed using the assignee's prior art process.

FIG. 2 is a top down view of the structure of FIG. 1, where FIG. 1 istaken across a horizontally bisected FIG. 2.

FIG. 3 is a perspective view of a single 3-terminal transistor that hasbeen singulated from a wafer, in accordance with one embodiment of theinvention. The transistors are mixed into a solution to form an ink forprinting on a substrate.

FIG. 4 is a cross-section of a small portion of a printed layer of thetransistors of FIG. 3 connected in parallel using three planes ofconductor layers. About 10 of the parallel-connected transistors may beprinted in each separate group, and an array of the groups is printedover the substrate.

FIG. 5 illustrates how the transistor of FIG. 3 may be an npn bipolartransistor.

FIG. 6 illustrates how the transistor of FIG. 3 may be a p-channelMOSFET.

FIGS. 7 and 8 illustrate how some transistors may be “incorrectly”interconnected by improper orientations of the transistors whenprinting, where the interconnections do not adversely affect thefunctions of the properly oriented transistors.

FIG. 9 is a chart identifying preferred functions of the top, bottom,and intermediate electrodes for MOSFETs and bipolar transistors.

FIG. 10 illustrates interconnecting groups of transistors to form alogic circuit.

FIG. 11 illustrates how the leads for the various groups of devices maybe brought to a patch area of the substrate for interconnecting thegroups. In another embodiment, the leads of logic gates (e.g., NANDgates) made from the groups may be brought to the patch area.

FIG. 12 is a top down view of conductors being patterned using ahydrophobic mask.

FIG. 13 is a cross-sectional view of one of the conductor lines formedusing the mask of FIG. 12.

FIG. 14 illustrates how the hydrophobic mask may be used for formconductor lines to interconnect groups of devices to form logiccircuits.

FIG. 15 illustrates how resistor values may be determined by thelocation of a conductor contacting printed resistive material.

FIG. 16 illustrates how the devices may be printed on both sides of asubstrate and interconnected by a through via.

FIG. 17 illustrates how circuits may be printed on relatively smallsubstrates and tested, then the small substrates are attached to alarger “base” substrate during a customizing step.

FIG. 18 illustrates how electrodes on the small substrate of FIG. 17 maybe bonded to electrodes on the base substrate.

FIG. 19 illustrates a roll-to-roll process that may be used to form thecircuits.

Elements that are similar or identical in the various figures arelabeled with the same numeral.

DETAILED DESCRIPTION

The printed programmable circuits of the present invention may use anycombination of passive devices (e.g., capacitors, resistors), 2-terminalinorganic semiconductor devices (e.g., diodes), and 3-terminal inorganicsemiconductor devices (e.g., transistors). The most complex device toprint and electrically connect to is a 3-terminal device. In some cases,a 3-terminal device, such a bipolar transistor, may be used as a diodeby only using two of the terminals or connecting two terminals to thesame conductor.

The 3-terminal devices used in embodiments of the present invention maybe less than the diameter of a human hair, rendering them essentiallyinvisible to the naked eye when the devices are sparsely spread across asubstrate. The sizes of the devices may range from about 10-200 micronsacross. The number of micro-devices per unit area may be freely adjustedwhen applying the micro-devices to the substrate. The devices may beprinted as an ink using flexography, screen printing, or other forms ofprinting. Conventional designs for 3-terminal devices may be easilyadapted for forming the micro-devices of the present invention. Theprecision of photolithography is well within the precision needed toform the micro-devices. Since many of the micro-devices will beoperating in parallel, the efficiency of each micro-device is notcritical.

FIG. 3 is a perspective view of a 3-terminal device 40 that can besuspended in a solvent and printed as an ink on a substrate. The device40 may be a bipolar transistor, a MOSFET, a JFET, a tri-MOS device, orany other 3-terminal device, generally including two current carryingterminals and a control terminal. The device 40 can be a lateral orvertical transistor, since the positions of the three electrodes do notdictate the locations of the semiconductor layers/regions or gatesinside the device 40. The electrodes may contact any location in thedevice 40 using vias.

The devices 40 are completely formed on a semiconductor wafer, includingthe electrode metallizations, by using one or more carrier wafers duringthe processing to gain access to both surfaces for metallization.Although the growth wafer may be silicon, the carrier wafer may be anymaterial. The silicon wafer is affixed to the carrier wafer using anadhesive or other suitable material. The shape of each device 40 isdefined by masking and etching. Various layers or regions may be dopedusing masked implantation or by doping the layers while beingepitaxially grown. After the devices are formed on the wafer, trenchesare photolithographically defined and etched in the front surface of thewafer around each device 40 down to the adhesive layer. A preferredshape of each device 40 is hexagonal. The trench etch exposes theunderlying wafer bonding adhesive. The adhesive is then dissolved in asolution to release the devices 40 from the carrier wafer. Singulationmay instead be performed by thinning the back surface of the carrierwafer until the devices 40 are singulated. The microscopic devices 40are then uniformly infused in a solvent, including a viscosity-modifyingpolymer resin, to form an ink for printing, such as screen printing orflexographic printing.

A similar technique may be used to form 2-terminal devices, such as avertical diode, with one electrode on top and another electrode on thebottom. The diode may have a shape similar to that shown in FIG. 3 butwithout the middle electrode.

Details regarding shaping vertical LEDs (2-terminal devices) in a waferand then singulating the LEDs for printing as an ink are described in USapplication publication US 2012/0164796, entitled, Method ofManufacturing a Printable Composition of Liquid or Gel Suspension ofDiodes, assigned to the present assignee and incorporated herein byreference. One skilled in the art may adapt such processes for formingthe 3-terminal device 40 and non-LED diodes.

The device 40 has two sections: a lower section 42 (or base portion) andan upper section 44. The upper section 44 is made relatively tall andnarrow so that the devices 40 are rotated in the solvent, by fluidpressure, as they settle on the substrate surface. The devices 40 rotateto an orientation of least resistance. Over 90% like orientation hasbeen achieved, although satisfactory performance may be achieved withover 75% of the devices 40 being in the same orientation.

The lower section 42 should be shaped so that the device 40 sits flat onthe substrate after the ink is cured. FIG. 4 illustrates three printeddevices 40, where only two are printed with the correct orientation.

The device 40 includes a metal top electrode 46, a metal intermediateelectrode 48, and a metal bottom electrode (not shown in FIG. 3). Theshape of the intermediate electrode 48 provides a large side surfacearea for good electrical contact with an intermediate conductor layer.

The intermediate electrode 48 should be offset with respect to themiddle of the device 40 so that an improper orientation of the device 40after printing results in the intermediate electrode 48 not electricallycontacting the intermediate conductor layer. In the example, theintermediate electrode 48 is below the middle of the device 40 (i.e.,H2<½ H1).

In FIG. 4, a starting substrate 50 is provided. The substrate 50 ispreferably thin and flexible for light weight, low cost, good heatconduction to air or a heat sink, and ease of processing. The substrate50 may be a suitable polymer, such as polycarbonate, PMMA, or PET, andmay be flexible so as to be dispensed from a roll. The substrate 50 canbe any size suitable for the ultimate product. The substrate 50 may be aconventional flex-circuit substrate, where metal (e.g., copper) traceshave been already formed on the substrate 50 by conventional means priorto the below processing steps.

If the substrate 50 does not already have metal traces formed on it as aflex-circuit, a conductor layer 52 (e.g., silver, aluminum, copper) isdeposited on the substrate 50 such as by printing. Conductive vias 54through the substrate 50 may be used to couple the conductor layer 52 toa metal layer 56 formed on the bottom surface of the substrate 50. Inthe various examples, the conductor layer 52 is printed as an array ofcircular spots on the substrate 50 (see FIG. 11). The spots areelectrically isolated from one another to allow groups of the devices 40to be interconnected in any manner to form logic circuits. Instead ofcircular spots, the conductor layer 52 may be printed in square spots orother shaped spots.

The devices 40 are then printed on the conductor layer 52 such as byflexography or by screen printing with a suitable mesh to allow thedevices 40 to pass through and control the thickness of the layer.Because of the comparatively low concentration, the devices 40 will beprinted as a loose monolayer and be fairly uniformly distributed overthe conductor layer 52. The printed locations of the devices 40 alignwith the locations of the printed spots of the conductor layer 52.

The solvent is then evaporated by heat using, for example, an infraredoven. After curing, the devices 40 remain attached to the underlyingconductor layer 52 with a small amount of residual resin that wasdissolved in the ink as a viscosity modifier. The adhesive properties ofthe resin and the decrease in volume of resin underneath the devices 40during curing press the bottom electrode 58 against the underlyingconductor layer 52, making ohmic contact with it.

A thin dielectric layer 60 is then printed to cover the conductor layer52 and further secure the devices 40 in position. The dielectric layer60 is designed to self-planarize during curing, by surface tension, soas to pull off of or de-wet the top electrode 46 and the intermediateelectrode 48. Therefore, etching the dielectric layer 60 is notrequired. If the dielectric layer 60 covers the electrodes 46/48, then ablanket etch may be used to expose the electrodes 46/48.

An intermediate conductor layer 62, aligned with the spots of theconductive layer 52, is then printed over the dielectric layer 60 toelectrically contact the intermediate electrode 48 and is cured in anoven appropriate for the type of conductor being used. The variousconductor layers may be metal (or contain metal) or be any other type ofprintable conductor layer.

Another thin dielectric layer 64 is printed over the intermediateconductor layer 62 so as not to cover the top electrode 46.

A top conductor layer 66, aligned with the spots of the intermediateconductor layer 62, is then printed over the dielectric layer 64 toelectrically contact the top electrode 46 and is cured in an ovenappropriate for the type of conductor being used.

A thicker metal layer 68 may then be printed over the conductor layer 66for improving electrical conductivity and/or heat conduction. Theintermediate conductor layer 62 extends out from the edge of the spot toform a terminal of the group of the devices 40.

FIG. 4 illustrates that the only steps needed to form the structure ofFIG. 4 are printing and curing steps 67 and 69. The random pattern ofthe devices 40 may resemble the pattern of LED 16 in FIG. 2.

FIG. 4 illustrates that the rightmost device 40A is oriented in theopposite direction. However, the intermediate electrode 48 is leftfloating, so the device 40A does not operate and has no effect on theresulting circuit.

The printed devices 40 are connected in parallel by the conductorlayers. Suitable operating voltages and control voltages are applied tothe conductor layers to operate the devices 40. In the example of FIG.4, the top electrode 46 is the control electrode for the devices 40(e.g., for gates or bases). The remaining two electrodes are the currentcarrying electrodes (e.g., source/drain, emitter/collector). Since theintermediate electrode 48 of the improperly oriented device 40A isfloating, the device 40 remains off and an open circuit.

FIG. 5 illustrates how the device 40 may be an npn bipolar transistors40B, where the intermediate electrode 48 is the base electrode. Theintermediate electrode 48 may be connected to any of the othersemiconductor layers in the device 40B using a via.

FIG. 6 illustrates how the device 40 may be a p-channel MOSFET 40C,where the intermediate electrode 48 is the source electrode. Theintermediate electrode 48 may be connected to any of the other layers inthe device 40C using a via.

If the devices 40 are to be connected as diodes, only conductor layers62 and 52 or 66 and 62 may be used. Accordingly, the effective polarityof the diode may be selected by which two conductor layers are used tocontact the diodes. Alternatively, two conducting layers may beconnected remotely to form a diode.

Any number of the devices 40 may be connected in parallel in a group forhandling a wide range of currents. In one embodiment, about 10 devices40 are located in each group. The groups of the devices 40 are printedas a 2-dimensional array of groups, such as by using a pattern on aroller in a flexography print process or by using a mask on a screenprint mesh, and the various conductor layers may be similarly patternedso that the devices 40 in each group are connected in parallel, but eachgroup is electrically isolated from one another. Therefore, each groupforms a separate component. The groups may then be selectivelyinterconnected using “programming” conductor traces on the substrate 50to form more complex circuits, such as logic circuits. A metalflex-circuit pattern on the substrate 50 may be used to interconnect thegroups of devices 40 for form the logic circuits. In one embodiment,since each group may be as small as a millimeter per side or amillimeter in diameter, a 2-dimensional array of such groups may exceedseveral thousand groups. Groups within a small area may beinterconnected to form logic gates, and the terminals of the gates maybe interconnected during programming to perform any logic function.

FIG. 7 illustrates how the improper orientation of the device 40A inFIG. 4 does not adversely affect the operation of the properly orienteddevices 40 in the group connected in parallel. The device 40/40A isassumed to be an npn bipolar transistor with a top electrode 46 for thebase, a bottom electrode 58 for the emitter, and an intermediateelectrode 48 for the collector. Since the device 40A is undesirablyoriented upside down during printing (shown in FIG. 4), its base isshorted to the emitter of the device 40 and its emitter is shorted tothe base of the device 40. When the base/emitter junction of the device40 is forward biased to turn on the device 40, the device 40A remainsoff and has no effect on the operation of the device 40. Note that, byusing an intermediate electrode 48 that is offset from the middle of thedevice 40 (as shown in FIGS. 3 and 4), the intermediate electrode 48 ofthe device 40A would be floating, making its effect even moreinsignificant.

FIG. 8 is similar to FIG. 9 but the devices 40 and 40A are MOSFETs.

FIG. 9 is a table showing possible connections for the top, bottom, andintermediate electrodes of the device 40 formed as a MOSFET or a bipolartransistor so that improper orientations do not adversely affect thefunctions of the properly oriented devices 40 connected in parallel.

FIG. 10 illustrates two groups, 72 and 74, of printed npn bipolartransistors (e.g., devices 40), where the transistors in each group areconnected in parallel so that each group acts as a single transistor.The print patterns for the devices 40 and conductor layers form thegroups as circular spots, but any shaped spot may be used. Theinterconnections of the groups in FIG. 4 cause the circuit to be an ANDgate. The conductive traces 75 connect to the various conductor layersin FIG. 4 for each group. The two transistors (i.e., groups 72 and 74)are connected in series between the supply voltage terminals 76 and 78,the bases of the transistors are connected to the input terminals 80 and82, and the output terminal 84 is connected to the emitter of thetransistor formed by group 74. The various terminals may be near theedges of the substrate 50 or proximate to the groups.

Resistors r1 and r2 are shown connected between the input terminals80/82 and the bases for current control. Due to the simplicity ofresistors, the resistive material may be directly patterned on thesubstrate with a patterned roller using flexography or a mask on ascreen mesh used to print the resistive material. Either the shape ofthe resistive material may determine the resistance or the position ofthe connector along its length may determine the resistance. A resistormay also be included on each device 40. Capacitors may also be formed byprinting the layers of the capacitors.

The substrate 50 may contain hundreds or thousands of such AND gates, orother gates, and the gates may be interconnected to form more complexfunctions. In such a case, the gates are equivalent to a programmablegate array. For a more flexible circuit, the groups may be initiallyunconnected, and the programming mask for the interconnections maydetermine the final circuit. Three-dimensional programming may be usedto allow the crossing over of traces. Any combination of gates and otherlogic circuitry may be created. Some groups may include transistors andother groups may contain other devices, such as diodes. Analog circuitsmay also be formed by interconnecting the various groups.

Due to the random but substantially uniform distribution of the devices40 in the ink, each group of the same area will have approximately thesame number of devices 40. Minor differences in the number of devices 40in a group will not affect the performance of a logic circuit. In oneembodiment, there may be about 10 identical devices in each group due tothe low currents required. The cost of the devices 40 in a single group,representing a single transistor, is about 0.143 cents. So the resultingcircuit board may be made relatively inexpensively.

As shown in FIG. 11, to simplify the programming of the groups, whichmay be printed in an ordered 2-dimensional array, conductive traces 85,originating at the conductor layers (FIG. 4) for all the groups, mayterminate at a patch area 86 on the substrate 50, where the product isnow a programmable circuit board 87. These traces 85 may be part of the“standard” design of the circuit board 87, which is then latercustomized for a particular use. This enables the printing process forforming the traces 85 to be optimized for connection to the conductorlayers in the groups and the programming process to be optimized forinterconnecting the ends of the terminals 88. For example, theprogramming process may be performed at a time after the circuit board87 has been fabricated, and the programming step may be performed byspecial equipment under computer control. Further, the pattern of theinterconnections may be much more complex than the traces 85 thatelectrically connect the transistor terminals to the patch area 86.

In the example of FIG. 11, the programming in the patch area 86 formsthe AND gate of FIG. 10. For more complex circuits, the programmingtraces 90 may need to cross, and multiple layers may be formed to avoidshorting of the traces.

In another embodiment, the groups of devices 40 may be initiallyinterconnected proximate to the groups to form separate logic gates,such as AND, NAND, NOR gates, and the leads for each gate terminate inthe patch area 86 for later programming to customize the substrate for aparticular customer. Accordingly, the generic circuit forms aprogrammable gate array.

A plurality of spaced patch areas may be provided on the circuit board87 to simplify routing of the interconnections. In one embodiment, theterminals for all the input signals are provided on one level in a patcharea, and the output terminals are provided on another level.

If the programming of the interconnections is complex, directly printingthe interconnections in an X-Y plane on the substrate 50 may beinsufficient. Direct printing of conductors on the substrate islimiting, since a minimum spacing between conductors is about 30 micronsto avoid cross-bridging, and thin conductors have a tendency to break upby surface tension.

In situations where it is not desired for the conductor lines to bedirectly printed, a mask layer is first formed on the substrate followedby the deposition of the conductor ink over the mask layer as follows.

One approach to pattern the interconnection traces, or to pattern anyother traces on the circuit board 87, or to pattern the groups ofdevices 40, is to form hydrophobic masks. The masks may be deposited byprinting (e.g., using a patterned roller or screen printing) or may bepatterned by a photolithographic process (if printing cannot achieve thedesired precision). One suitable masking substance is thoroughly cleaneddiatomaceous earth particles infused in a solution as an ink. The ink isprinted in a pattern that is negative to the desired wiring/devicepattern. After curing, the resultant film is activated via afluorination process, yielding a super-hydrophobic surface (i.e., itwill not wet by the conductor ink or the device ink). The area of thesubstrate that is exposed by the film will be either mildly hydrophilicor super-hydrophilic (i.e., it will wet by the conductor ink or thedevice ink).

For forming the traces, a hydrophilic conductive ink is prepared anddeposited over the hydrophobic mask. The exposed substrate areas will becovered by the ink, and the conductive ink that has been deposited onthe hydrophobic mask surface will pile up in the exposed areas. Thisyields a greater cross-sectional area of the conductive ink (for goodconductivity and mechanical strength) and prevents cross-bridges.

FIG. 12 is a top down view of the hydrophobic mask 94 defining areas 96exposing the substrate. FIG. 13 is a cross-sectional view showing asingle conductor 98 formed in one of the areas 96. Note that theconductor 98 is thicker than the mask 94. The height of the conductor 98is determined by the amount of conductive ink deposited over the mask.For masks defining a large exposed area of the substrate, moreconductive ink needs to be deposited to ensure that the exposed areasare fully covered by the ink. At termination areas of the traces, suchas for connecting the ends of the traces to other conductors, anexpanded pad area should be formed to ease alignment tolerances for asubsequent printed layer and to improve the resulting electricalconnection.

After curing the conductive ink, a dielectric ink is then deposited overthe same mask, where the dielectric ink contains sufficient surfactantto cover the mask surface and the conductors and neutralize thehydrophobic effects of the mask. Additional mask and trace layers may beformed to create a 3-dimensional matrix of interconnections. Verticalvias may be used for interconnections between conductor layers.

FIG. 14 illustrates the use of a hydrophobic mask 100 over the circuitboard 87 of FIG. 11 when creating the interconnections between groups 72and 74 to create an AND gate. In another embodiment, the mask 10 is onlyused in the patch area 86 for programming, and the traces 85 leading tothe various groups are formed when printing the various conductor layersfor the groups.

This general masking process may also be used for patterning the groupsof devices 40 and conductor layers. Groups of the same or differentdevices may be stacked to allow the formation of very complex circuits.

The programming process may be inexpensively performed in a roll-to-rollprocess on a large number of the flexible circuit boards 87 after thestandard features of the circuit boards 87 have been formed. After thefinal programming, the circuit boards 87 may be singulated from theroll. As seen, no vacuum processing or dangerous materials are used tofabricate the circuit board 87 and program it.

FIG. 15 illustrates how the resistors R1 and R2 in FIG. 14 may be formedby flexography or screen printing the resistive material on thesubstrate, where a mask on the screen mesh defines the shapes of theresistive material. Other deposition techniques may be used. Either theshape of the resistive material (length, width, height) may determinethe resistance or the position of the connector 102 or 103 along itslength may determine the resistance. If the position of the connectordetermines the resistance, all the resistors may be formed identically.The resistance can also be selected by interconnecting resistors inseries and/or parallel.

FIG. 16 illustrates both a bottom view and a cross-sectional view of acircuit board 106 where both sides of the substrate 108 have printedthereon the groups of devices, such as groups 72 and 74 on the bottomand other groups 110 and 112 on the top. The traces 114 and 116interconnect the groups. Through vias 118 connect the circuits on oneside to the circuits on the other side. Prior to printing theinterconnect layer, the via holes are punched in the substrate 108 andfilled with a UV-cured hole-filling conductor, for example. If mirrorimages are formed, this simplifies the interconnection design, since thepatch areas on both sides can be the same.

Instead of vias, a wrap-around connector can be used.

Since the substrate 108 may be a very thin and flexible film (like aflex-circuit), the resulting circuit board 106 may be folded to reduceits size. Flexible conductors formed by inks are commercially available.There may be special areas on the substrate 108 that define where thecircuit board 106 may be folded without damage to the circuitry.

To improve reliability and flexibility in the usage of the circuitboards, a “base” circuit board 120 (FIG. 17) may be fabricated to havecertain basic features and connection terminals. After the circuit board120 has been tested and approved, additional circuit boards 122 and 124may be electrically attached to the base circuit board 120 to customizethe performance for a particular application.

In the embodiment of FIG. 17, the tested and approved circuit boards 122and 124 have an adhesive applied to its surface that will be adhered tothe base circuit board 120. The terminals 126 of the circuit boards 122and 124 align with terminals on the base circuit board 120. Thoseterminals 126 are coated with a conductive adhesive. The circuit boards122 and 124 are then aligned with the base circuit board 120 and adheredto the surface of the base circuit board 120. In one embodiment, the“device side” of the circuit boards 122 and 124 face the device side ofthe base circuit board 120. By forming various functional unitsseparately, the pass rate for each unit during testing will be higher,and the functional units can be connected in various combinations to addmore functional possibilities.

In one embodiment, the circuit boards 122/124 are formed in aroll-to-roll process and, after testing, the adhesives are applied atthe final station. The circuit boards 122/124 may have test tabs thatare cut during singulation. After singulation, the circuit boards122/124 are adhered to the base circuit board 120. As an arbitraryexample, one circuit board 122 may be an A/D converter and the other onemay be a D/A converter.

FIG. 18 illustrates another technique for mounting the circuit board 122to the base circuit board 120. In FIG. 18, the circuit board 122 isperforated at the electrical connection locations at areas 128. Thebottom side (non-device side) of the circuit board 122 is then coatedwith a dielectric adhesive, and the circuit board 122 is adhered to thebase circuit board 120 so the perforations are over the connectionterminals on the base circuit board 120. A conductive adhesive 130 isthen deposited through the perforations to connect the terminals of thebase circuit board 120 to the top terminals of the circuit board 122.For example, the traces 132 and 134 are connected by the conductiveadhesive 130.

This technique can also be used with the double-sided circuit board ofFIG. 16.

Using massively redundant arrays of devices (e.g., devices 40 in FIG.11), along with the standard passive devices (e.g., resistors R1-R3 inFIG. 11) in the patch areas 86, allows the circuit boards to have a veryhigh pass rate and creates programmable circuit boards that can be laterprogrammed to make unique devices on an as-needed basis.

For higher densities of the groups of the devices, multiple insulatedlayers of the groups may be printed to form a 3-dimensional structure.Vertical vias may be used to gain access to the various layers. Groupsof the devices may be connected in series using vertically alignedgroups.

FIG. 19 schematically illustrates one possible assembly line formanufacturing the circuits by printing in a roll-to-roll process. Theroll 136 contains the substrate material, and the roll 138 is a take-uproll. The various stations are labeled. The process sequentially printsthe various layers and cures the layers. Flexography is preferred forprinting using a roll-to-roll process. The number of layers depends onthe complexity of the circuits and the devices printed. The roll-to-rollprocess may produce the unprogrammed circuit boards, and a separatesystem may be used for the final programming step, depending on aparticular customer need.

The various directional attributes used herein, such as bottom, top, andvertical, are not to be construed to convey absolute directions relativeto the Earth's surface but are used to convey orientations relative tothe enclosed figures when the drawing sheets are held upright. In anactual embodiment, such terms still apply to the product regardless ofthe absolute orientation of the product relative to the Earth's surface.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A method for forming a circuit comprising:providing an ink containing a plurality of pre-formed, semiconductorelectrical devices mixed in a solvent, each of the devices having atleast a first electrode and a second electrode; printing the ink, on asubstrate, to form a plurality of separate groups of the pre-formed,semiconductor electrical devices, each group containing a plurality ofsubstantially identical electrical devices forming a randomtwo-dimensional arrangement of the electrical devices on the substrate;forming at least one conductor layer to connect the electrical devicesin each group in parallel; and interconnecting at least some of thegroups, using an interconnection pattern, to achieve an electricalfunction.
 2. The method of claim 1 wherein each of the electricaldevices include three electrodes.
 3. The method of claim 1 wherein theelectrical devices include at least one of transistors or diodes.
 4. Themethod of claim 1 wherein the step of interconnecting at least some ofthe groups comprises interconnecting at least some of the groups tocreate logic gates.
 5. The method of claim 1 further comprising formingat least one input conductor for receiving an input signal for theelectrical function, and forming at least one output conductor foroutputting an output signal as a product of the electrical function. 6.The method of claim 1 wherein the step of interconnecting at least someof the groups comprises interconnecting the groups in a designated patcharea.
 7. The method of claim 1 wherein the step of interconnecting atleast some of the groups comprises interconnecting the groups using afirst interconnection pattern to form logic gates and theninterconnecting the logic gates in a designated patch area using asecond interconnection pattern.
 8. The method of claim 1 wherein thecircuit is formed using a roll-to-roll process to form flexiblecircuits.
 9. The method of claim 1 wherein the electrical devices havelargest dimensions between 10-200 microns.
 10. The method of claim 1wherein the circuit forms a programmable gate array.